Natural gas, which is used by household consumers, is composed primarily of methane. Prior to use, it must be filtered, stripped of crude oil and other higher boiling alkanes (e.g. ethane, propane, or pentane), dehydrated, and “sweetened”, whereby CO2 and H2S are removed from the natural gas. Amongst these purification steps, both 1) the dehydration step and 2) the “sweetening” step could benefit significantly from using purification agents with increased surface area.
In the dehydration step, the gas is treated with ethylene glycols (mono-, di-, tri-, etc.) to remove water. Due to the high affinity of glycols for water, the wet gas that flows through a ‘contactor’, which is in essence a tower packed with mesh or bubble cap trays that maximize contact between the glycol (dessicant) and the gas, is dried through its interaction with the glycols.
The removal of acid gases from raw natural gas is referred to as “gas sweetening”. Raw natural gas typically contains far higher levels of CO2 and H2S than are allowed in the final product. The corrosive nature of the acidic gases and the toxicity of H2S are amongst the principle reasons for minimization of their contamination in natural gases. During the gas “sweetening” step, where H2S and CO2 are removed, the gas flows through an amine-containing contactor or through a column of agitated amine solution. The amine absorbs and reacts with H2S and CO2, thus removing the acidic gases from the gas mixture.
In both steps, the contact of the purifying liquids with crude or unpurified natural gas is important. By enhancing the contact area of the liquids with the gas, the efficiency and rate of purification would increase. Further, the domain size of the liquids is decreased by discretizing the liquids into micronized droplets, thus enhancing mass transport of gases (in particular gases to be absorbed) into the sorbent. This would allow the usage of shorter purification columns/contactors and higher gas flow rates. Also, energy could be saved because there would be a decreased need for agitation of viscous liquids like diethanolamine (DEA) or monoethanolamine (MEA), which are widely used in gas sweetening processes.
In industrial gas purification set-ups, neat MEA is rarely used. Most commonly, a 20-30 wt % aqueous solution of MEA is utilized instead. This is due to the high viscosity of neat MEA (approximately 24 times that of water at 20° C.) as well as its corrosive nature. High liquid viscosity results in difficulty of liquid agitation and transport through the gas contactors. Neat DEA and triethanolamine (TEA) have even higher viscosity than that of MEA, and their agitation for enhanced surface interaction with the gases demands even more energy.
To address the above drawbacks, dry liquids, or otherwise known as micronized droplets of liquids, are used in gas purification steps, in particular, but not limited to, gas sweetening and dehydration of raw natural gases, and also the purification of flue or waste gases. See, for example, the contents of PCT Publication No. WO 2014/189470 entitled “Method For Purifying Gas Using Liquid Marbles”. Such dry liquids or liquid marbles are employed as sorbents to adsorb H2S and CO2, for example.
To make the gas purification process more economically feasible, it is desirable to regenerate or recover the used sorbents after having a gas adsorbate adsorbed thereto. As mentioned above, common amines used in the gas purification step are MEA and DEA, which are extremely viscous. Frequently, to reduce the viscosity of these amines, they are diluted with water. This imposes a considerable energy cost on removing carbon dioxide since much energy is used in heating up water (which has a high heat capacity) during the sorbent regeneration step.
For the removal of carbon dioxide from flue gases (and not just for natural gas processing), amine scrubbing is also a commonly used method. Upon adsorption of carbon dioxide by amines, carbamates are formed. The amine is regenerated by the cleavage of the C—N bond of the carbamate, removing carbon dioxide in the process. This is achieved by the vigorous heating of the carbamates.
However, the regeneration of gas sorbents poses various difficulties, particularly when they are in the dry liquid form. For example, in the regeneration of dry MEA, the monoethanolamine is progressively oxidized and some MEA is lost as vapor during the regeneration process due to the purge gas flow as well as the heat input.
Another limitation of highly porous materials such as powderized or dry liquid sorbents is the low efficiency of heat transfer throughout the material during the regeneration process.
In the case of bulk liquid sorbents, the process of heat transfer through the sorbents is largely by the three mechanisms of conduction, convection and radiation.
During the regeneration of the liquid sorbent, the sorbent is heated to induce desorption of adsorbed gases, whereby the gases may be either chemisorbed or physisorbed. However, in highly porous materials, because of the low efficiency of heat transfer, non-uniform heating of the sorbents may present a problem, particularly for large volumes of sorbents.
Therefore, there remains a need to provide for a cost effective and efficient method of regenerating used sorbents employed in gas purification processes.